Mobile networks typically have group management systems and/or group communication systems. Group management systems facilitate communication among group members. Several secure group communication systems have been built in the so-called “fortress model,” where the group members are assumed to be correct and use cryptography to protect their communication from external attackers. One such system, Ensemble, uses group key distribution protocols to distribute a shared group key, while another, Secure Spread, uses a contributory key agreement protocol in which every group member contributes an equal share of the group secret.
Group communication systems, which also facilitate communication, have been developed in the Byzantine fault model. In this model, faulty processes can fail arbitrarily. For example, the Rampart system and the SecureRing system provide services for membership and ordered message delivery, and they depend on failure detectors to remove faulty processes from the membership. They rely on synchrony for both safety and liveness, since inconsistency can arise if a membership is installed that has one-third or more faulty processes.
Over the last several years, much of the work in Byzantine fault-tolerant agreement has focused on Byzantine fault-tolerant state machine replication (SMR) protocols. In the state machine approach, a group of servers totally orders all updates that cause state transitions, and then the servers apply the updates in the agreed upon order. If the servers begin in the same initial state and the updates are deterministic, the servers will remain consistent with one another. SMR protocols provide strong consistency semantics, but they allow at most one partition to continue executing new updates at a time.
Threshold cryptography has been used to provide security in peer-to-peer and Mobile Ad-hoc Network (MANET) settings, such as the use of threshold cryptography for admission control in malicious environments. In one approach, the current group members run a voting protocol (based on a threshold digital signature scheme) to decide whether or not to admit a potential group member.